    \begin{tikzpicture}
    %------------------------------------------------------------------------------------
    %    图片2.8
        \node (E) at (0,0) {E}
        [level 1/.style={level distance=3.5em,sibling distance=3.5em},
        level 2/.style={level distance=3.5em,sibling distance=3.2em},]
            child {node (1E) {E}
                child {node {i}}
            }
            child {node {+}}
            child {node {E}
                child {node {E}
                    child {node (3i) {i}}
                }
                child {node {*}}
                child {node {E}
                    child {node {i}}
                }
            };
        
        \node (E2) at ([xshift=17em]E) {E}
        [level 1/.style={level distance=3.5em,sibling distance=3.5em},
        level 2/.style={level distance=3.5em,sibling distance=3.2em},]
            child {node {E}
                child {node {E}
                    child {node {i}}
                }
                child {node {+}}
                child {node {E}
                    child {node {i}}
                }
            }
            child {node {*}}
            child {node (1E2) {E}
                child {node {i}}
            };
        
        \begin{pgfonlayer}{background}
            \node (background) [fit=(E) (1E) (1E2) (3i)]{};
        \end{pgfonlayer}
        
        \draw (background.north) -- (background.south);
    \end{tikzpicture}